The present invention relates to high solids aqueous based pressure sensitive adhesives.
In recent years there has been a trend toward the reduction of the amount of solvents used in pressure sensitive adhesives. There have been a number of patents which disclose aqueous based pressure sensitive adhesives including U.S. Pat. Nos. 4,189,419 issued Feb. 18, 1980 assigned to Avery International Corporation; 4,508,864 issued Apr. 2, 1985 assigned to Polysar Limited; and 4,540,739 issued Sept. 10, 1985 assigned to Polysar Limited. Generally, this art discloses pressure sensitive adhesives which are a mixture of an emulsion of a polymer and a tackifier resin.
One of the difficulties with these types of systems is the incorporation of the tackifier into the polymeric emulsion. The tackifier resins may be dispersed in an organic solvent or diluent and mixed with the emulsion, then the organic solvent or diluent is driven off. While this is effective it defeats one of the purposes of going to aqueous based pressure sensitive adhesives, specifically to reduce such organic volatiles. The tackifier resin may be added to the polymeric emulsion in the form of an aqueous emulsion or dispersion. Such a procedure has a number of drawbacks. The blending of two aqueous dispersions tends to result in a mixture having a solids content in between that of the original dispersions. As a result the adhesives contain an undesirably high amount of water. Additionally, in preparing the tackifier emulsion surfactants are usually required. These surfactants usually remain in the resulting adhesive making it sensitive to moisture.
European Patent Application No. 0,084,829 published 03.08.83 in the name of Hercules Incorporated has attempted to address this problem. This patent application discloses a process for making a water dispersable tackifier resin comprising heating the resin and adding thereto from about 3 to 10 percent of a non ionic emulsifier. Preferably the non ionic emulsifier is a polyalkylene alkyl phenol, in which the polyalkylene radical comprises from 4 to 9 alkylene oxide units. The amount of emulsifier used in accordance with the present invention is lower than that disclosed in E.P. Application No. 0,084,829. Additionally, the present invention is directed to blending the tackifier directly with an emulsion of a polymer while the Hercules disclosure teaches that it is preferable to disperse the resin in water.